The problem of removing SO.sub.2 from stack gases has been confronted in the past by varied approaches. Gorin et al in U.S. Pat. No. 3,906,080 discloses an improved SO.sub.2 removal method wherein thiosulfate rich aqueous solution containing a sulfite forming additive is used as the selective SO.sub.2 absorbent wherein a gas is substantially reduced in SO.sub.2 content.
Hansen in U.S. Pat. No. 1,179,120 discloses a process for the simultaneous absorption of ammonia and hydrogen sulfide from industrial gases. Hansen uses a process having two stages, the first stage of which is an ammonium sulfite bisulfite wash having a lower ratio figure of sulfur dioxide to ammonia than about 1.5 and a second stage of which is an ammonium sulfite bisulfite wash having a higher ratio figure of sulfur dioxide to ammonia than about 1.5.
South African Pat. No. 74/4312 disclosed by Neville relates generally to processes for the selective and simultaneous removal of noxous gases, namely oxides of nitrogen and sulfur, from waste industrial gases. A distinct advantage of the process described is that an residual sulfites present in the solution leaving the scrubber are rapidly oxidized to sulfates. This reaction is strongly catalyzed by the presence of small quantities of NO coordinating salts (e.g. Mn, Cu, Fe).
Barnes et al in U.S. Pat. No. 4,130,628 discloses a method for scrubbing SO.sub.2 and NO.sub.x containing gases with iron sulfide.